Fermi Gamma-ray Space Telescope

Methods for Identifying Pair Halos

Brendan Wells
Steve Ritz, Bill Atwood

Abstract:

Very high energy gamma rays from distant active galactic nuclei (AGN) experience attenuation by interacting with extragalactic background photons and converting into e+e- pairs. With non-zero intergalactic magnetic fields, the electron and positron will be deflected as they propagate, gradually losing energy by upscattering cosmic microwave background photons. However, the visible consequences of these cascades, ``pair halos'' around the original source AGN, have not yet been directly observed. Based on a two-component approach to modeling the AGN core/halo image, we propose three new methods for determining the presence or absence of pair halos. Two depend on comparing flaring and quiescent AGN periods, and one depends on the ratio of components as a function of observation angle. The sensitivity of these methods is demonstrated through the use of a new, detailed halo simulation.